Method for the removal of deposited crude maleic acid



United States Patent 3,129,455 METHUD FQR THE REMQVAL (3F DEPDSETED RUDEMALEIC ACID Manfred Gans, Leonia, Nd assignor to Halcon llnternatienailine, a corperation of Delaware No Drawing. Filed Aug. 11, 1950, Ser.No. 48,843 1 Claim. (Cl. 13430) This invention relates to themanufacture of maleic anhydride, including the dehydration of maleicacid by distillation in the presence of a water azeotroping agent toform maleic anhydride, and especially to such a process wherein theformed maleic anhydride is supersaturated with crude maleic acid whichmay include fumaric acid and resins and tars which tend to settle outand form a tenacious strongly adherent coating on steel equipment andthe material is brought to or maintained at a temperature of at least185 C. to render the crude maleic acid relatively non-adherent, and moreparticularly to such a process wherein a formed adherent solid depositof the crude on the equipment is rendered relatively nonadherent byheating to a temperature to at least about 185 C. and then dispersed ina liquid such as water and removed.

Maleic anhydride is a commercially important material and it may beprepared by the catalytic partial oxidation of benzene or the likehydrocarbon in the presence of a catalyst such as vanadia or the likewhich may be supported on a carrier. The hot gaseous reaction mixturemay be cooled to partially condense some of the maleic anhydride, andthe remainder may be absorbed in Water to make maleic acid. The latteris converted to maleic anhydride by dehydration; e.g. by distillation inthe presence of an azeotroping agent such as xylene or the like. Themaleic anhydride obtained may contain some of the azeotroping agent andit may be stored for considerable time prior to further distillation toproduce specification grade product. The maleic anhydride may besupersaturated with crude maleic acid which may include a large portionof fumaric acid together with some resins and tars and this may settleout to form a tenacious and ad herent solid deposit on or in the steelequipment used. Removal of this deposit is a major manufacturingproblem, inasmuch as the solid is very tough and cannot be broken upeasily. In addition, attempts to remove it by means of high pressuresteam and water are extremely time consuming. Even the use of hot wateralternated with hot aqueous caustic has been found to be ineffective forremoval of the hard deposit formed in a storage tank.

The discoveries associated with the invention and relating to thesolutions of the above problems, and the objects achieved in accordancewith the invention as set forth herein include the provision of:

A process for rendering crude maleic acid non-adherent to steel whichmethod comprises maintaining the steel at a temperature of at least 185C. during contact thereof with a super-saturated solution or mixture ofthe crude acid in maleic anhydiide;

Such processes for removing a solid crude maleic acid deposit includingheating the deposit to a temperature of at least about 185 C. and thendispersing it in water maintained at a temperature of about 100 C;

Such processes wherein the deposit has been precipitated onto steelequipment from a solution thereof in maleic anhydride;

Such processes wherein the deposit is submerged in liquid during theheating step;

Such processes wherein the liquid is predominantly maleic anhydride;

Such processes wherein the heating is at a temperature in the range of185 C. to 200 C.;

And other objects which will be apparent as details or embodiments ofthe invention are set forth hereinafter.

In order to indicate still more fully the nature of the presentinvention, the following examples of typical procedures are set forth inwhich parts and percent mean part and percent by weight, respectively,unless otherwise indicated, it being understood that these examples arepresented as illustrative only and are not intended to limit the scopeof the invention.

Example 1 A benzene air mixture containing 1.21 mol percent of benzeneis fed downward through a tubular reactor at a volume space velocity of2500 per hour (volume of feed gas per volume of catalyst per hour), at areaction temperature of 365 C. (using a modified vanadia catalyst asdescribed in the Robert B. Egbert and Mitchell Becker Patent, 2,777,868,issued January 15, 1957).

The reactor tube may be of inch internal diameter and the catalyst bedheight may be 10 ft. The tube may be surrounded by a temperatureregulating medium such as molten salt, molten metal or a copper jacket.

The gaseous reaction mixture is passed through a partial condensercontaining one or more vertical tubes of about one inch internaldiameter and about 8 feet in length. The tubes are surrounded by atemperature regulating medium maintained at 53 to 58 C., e.g.circulating water. About one third of the maleic anhydride content ofthe gas is condensed and drains out as a liquid. The remainder of thegas is passed to a water scrubbing column or tower wherein it iscontacted countercurrently with Water or aqueous maleic acid, so as toprovide a 40 percent by weight aqueous maleic acid solution.

This solution is dehydrated in a column in the presence of xylene as anazeotroping agent (the distilled water being removed and the xylenebeing recirculated as reflux to the column). A crude maleic anhydridematerial is removed as bottoms, and may contain about 20 percent or morexylene.

If desired, a part of the bottoms from the distillation may be removedand passed to a cooling and settling tank, wherein it is subjected to avacuum so as to flash a part of the xylene and more maleic anhydride(which is condensed and recycled) and the temperature of the mixture islowered to about C. Solid crude maleic acid containing a substantialamount of fumaric acid settles out in such a treatment, and thesupernated liquid may be decanted and recycled to the distillationcolumn. This procedure is set forth more fully in co-pending applicationof Walter N. Alexander and Manfred Gans, entitled Chemical Process,Serial Number 44,687, filed on July 22, 1960, now abandoned.

The crude maleic anhydride material may contain up to about 50 percentxylene, and this material may be held in accumulation or storage tanks,and then subsequently refined or further distilled to make specificationgrade maleic anhydride. During the distillation, storage, and optionalsettling of crude acid, a tough adherent deposit may be formed. Afteraccumulation of the deposit is substantial, the particular piece ofequipment has to be removed or isolated from service and cleaned.

The invention is typified by the cleaning of a large horizontalcylindrical ordinary steel (or stainless steel) vessel used to store thecrude maleic anhydride for a period of several days or weeks until thesolid deposit therein occupies about 10-20 percent of the volume of thetank. The liquid is removed from the tank, and the residue is brought toa temperature of about C. using a heating coil supplied with steam at350 p.s.i.g. pressure and having a condensation temperature of about 225C. Then water is introduced into the tank (while heating of the coil iscontinued) so that the temperature in the tank is at about 100 C. Thesticky hard solid deposit goes into dispersion or solution into thewater rapidly with no agitation beyond that brought about by introducingthe water and the inherent boiling thereof. Then the tank is drained,and may be washed with Water at about 100 C. As a result, the tank iscleaned in about two hours.

To remove the liquid before heating it may be drained or if the take-oilnozzles are plugged, a siphon may be used. Alternatively, the liquid isboiled out while heating the tank contents.

In a comparative run, except without preheating to about 190 C.,repeated hot washes with Water at 100 C. do not remove the solid.

Example 2 The procedure of Example 1 is repeated, except that a seriesof steel pipes used to convey the crude maleic anhydride is maintainedat a temperature of at least about 185 C. by means of a steam jacketsupplied with 300 p.s.i.g. steam. These pipes show no fouling orformation of tough adherent solid deposits.

If flow of liquid is stopped for up to about ten hours, solid settlesout, but it is removed promptly when flow is started up again.

In a comparative run, Example 2 is repeated except that the heating ofthe pipes is by means of copper tubing wrapped around the pipes andsupplied with steam so as to maintain the pipe at a temperature of about130 to 150 C. The pipes are gradually coated with tough adherent solids,and great difiiculty is experienced in removing these solids.

The preheating should be at a temperature of at least about 185 C. andabout 200 C. is a practical upper limit, although higher temperaturesmay be used. The water is applied when the solid reaches thistemperature, but a heating at this temperature for about thirty minutesbefore adding the water is preferred.

Any convenient heating means may be used, including a heating jacket ora heating coil (steam); of course, severe charring or coking is to beavoided.

The crude deposit is removed from steel (including the ordinary andstainless types) or other materials of construction used in chemicalplants, which materials can Withstand the indicated temperatures.

The water azeotroping agent may be any such agent which is compatible inthe system including xylene, toluene, and the like hydrocarbon, orhalogenated or oxygenated hydrocarbon having equivalent solubility anddistillation properties.

It is indeed surprising that crude maleic anhydride may be produced withhigh efiiciencies and that equipment in which it is stored can becleaned rapidly and that the deposit of solids on walls of pipinghandling this crude can be avoided especially in view of experience withextremely troublesome deposits of crude maleic anhydride includingfumaric acid, resins and tars in transmitting lines or storage tanks. 1

In view of the foregoing disclosures, variations and modificationsthereof will be apparent to one skilled in the art and it is intended toinclude within the invention all such variations and modificationsexcept as do not come within the scope of the appended claim.

What is claimed is:

A method of removing deposits containing maleic acid, fumaric acid,resins and tars from steel, said deposits being formed in the presenceof a liquid containing maleic anhydride which comprises: separating saidliquid from said deposits; heating said deposits to a temperature of atleast 185 C. and thereafter dispersing said deposits by contact withWater at a temperature of about 100 C.

References Cited in the file of this patent UNITED STATES PATENTS2,662,041 Dougherty et al Dec. 8, 1953 2,917,418 Cathcart Dec. 15, 19593,024,251 Feder et al Mar. 6, 1962 OTHER REFERENCES Handbook ofChemistry and Physics, 28 ed., 1944, Chemical Rubber Publishing Co.,Cleveland, Ohio (pp. 868-869 relied on). (Copy in Sci. Library.)

